(1) Field of the Invention
The present invention relates to a load driving circuit and a multi-load feedback circuit; in particular, it relates to a load driving circuit and a multi-load feedback circuit used to drive plural Light Emitting Diode strings.
(2) Description of the Prior Art
Refer first to FIG. 1, wherein a schematic diagram of a conventional constant current driving apparatus for LEDs is shown. The illustrated LED constant current driving apparatus comprises a current balancing circuit 10, a LED module 60 and an electrical power supply 70. The electrical power supply 70 stabilizes the output voltage VOUT through a voltage feedback signal VFB generated by a voltage feedback circuit. The LED module 60 has plural LED strings connected in parallel between the electrical power supply 70 and the current balancing circuit 10. The current balancing circuit 10 has a current setting resistor 11 as well as a current mirror composed of a transistor 12 and multiple transistors 20. One terminal of the current setting resistor 11 is coupled to a voltage VCC, and the other terminal thereof coupled to the transistor 12, thereby allowing a setting current to flow through the transistor 12. The transistor 20 is one-to-one, individually connected to a corresponding LED strings in the LED module 60, and mirrors the setting current, thereby allowing the setting current to flow through the LEDs for light emissions. In this way, substantially equal current can flow through each LED in the LED module 60 for substantially emitting same brightness.
Due to significant differences in threshold voltages between the LEDs, the required driving voltage value to maintain the same current may vary. For example, with a current of 20 mA flowing therethrough, the required driving voltage for one single LED is roughly within a range of 3.4˜3.8V, and each LED string in the LED module 60 has 20 LEDs, the required driving voltage for one LED string is accordingly within a range of roughly 68˜76V, and the difference in the difference of driving voltage between each series of LEDs is endured by the transistor switch 20. Besides, the transistor switch 20 must operate in the saturation range to mirror current. Therefore, to ensure each LED string to acquire the same current flowing therethrough, the output voltage VOUT provided by the electrical power supply 70 must be higher than the maximum driving voltage, e.g., 80V, thereby ensuring the transistor switch 20 to operate in the saturation range.
Nevertheless, the driving voltages required by the LED strings is unlikely to be individually confirmed beforehand, so the maximum driving voltage for the LED strings in the LED module 60 may be lower than 76V. As a result, excessive provision of 80V as the driving voltage may contrarily cause reduced illumination efficiency. Furthermore, to prevent LED string from open-circuit due to any LED damage in the LED string, the LED can be connected in parallel to a Zener diode, such that current can be successfully bypass through the Zener diode when the LED is damaged. The breakdown voltage in the Zener diode is set to be higher than the threshold voltage of LED, e.g., 2V., so as to prevent occurrences of erroneous actions in the Zener diode. Under such circumstances, if two LEDs are damaged in the same LED string, thus resulting in approximately 4V increments in the driving voltage of the LED strings, it is possible to lead to significant reduction in the current flowing through the LED strings or even no current. Alternatively, to increase the output voltage VOUT provided by the electrical power supply 70 to keep the amount of current, illumination efficiency may be undesirably lowered.